1. Field of the Invention
The present invention relates to an inkjet recording apparatus, a test image forming method and a computer-readable medium, and more particularly to an in-line inspection and an ejection abnormality determination technology for an image determination apparatus of an inkjet recording apparatus or the like.
2. Description of the Related Art
Conventionally, it is suitable to use an inkjet recording apparatus as a general image forming apparatus. An inkjet recording apparatus forms a color image on a recording medium by ejecting colored inks of black, cyan, magenta, yellow, and the like, from a plurality of nozzles provided in an inkjet head. However, in an inkjet recording apparatus, if an ejection abnormality occurs, such as an ejection failure in which ink is not ejected, an abnormality in the direction of flight, an abnormality in the ejection volume, or the like, then the quality of the image declines markedly. In particular, in single-pass image forming using a full line head having nozzle rows of a length corresponding to the full width of the recording medium, if an ejection abnormality such as that described above occurs, then a white stripe following the conveyance direction arises in the recording medium and marked decline in quality occurs in the recorded image. Various methods for judging the presence or absence of ejection abnormalities in an inkjet recording apparatus have been proposed.
For example, there is a method which uses an in-line inspection apparatus using a CCD (Changed Coupled Device) having a structure in which photocells are arranged in a direction perpendicular to the scanning direction (conveyance direction) of the recording medium that is the object of inspection, a test pattern is printed on the recording medium, the test pattern is read in by the in-line inspection apparatus, and ejection abnormalities are judged from the read results.
The line inkjet printer disclosed in Japanese Patent Application Publication No. 2004-9474 is an inkjet printer which carries out printing with a fixed print head having a greater width than the printing width of the printing paper, having a composition whereby a test pattern printed in a portion of the paper by displacing the ejection nozzles by a uniform interval is read in by a scanner unit, and checking of ejection failures for all of the nozzles is carried out for each plurality of pages or each page.
However, in order to judge accurately the presence or absence of an ejection abnormality for each nozzle, it is necessary to prepare a line CCD having a higher reading resolution than the print resolution, but a high-resolution line CCD requires a longer time to read in the determination signal than a low-resolution line CCD, and hence there are concerns about decline in the determination efficiency. Furthermore, a high-resolution line CCD is expensive and is therefore unbeneficial from the viewpoint of cost. On the other hand, if a low-resolution line CCD is used, there may be a plurality of dots in the determination area of one element and therefore it is extremely difficult to determine ejection for each respective nozzle (each dot).
In the invention described in Japanese Patent Application Publication No. 2004-9474, the image is read in by a scanner unit having a reading resolution which is equal to or higher than the printing resolution, and Japanese Patent Application Publication No. 2004-9474 makes absolutely no mention of a case where a scanner unit having a lower reading resolution than the printing resolution is used.
In other words, Japanese Patent Application Publication No. 2004-9474 discloses a method in which a test pattern of vertical lines in a 1-on N-off arrangement (N=natural number) (namely, a test pattern comprising a plurality of lines extending in the conveyance direction of the paper printed by displacing the print nozzles) is printed in one portion of the paper, and the test pattern is read in and binarized; when the line scanning rate is 1 (msec/Line) and the paper feed rate is 1 (m/sec), then the width of the pattern is calculated to be 1 mm (=1(m/sec)×0.001 (sec/Line)), but it is not stated what value N is set to in cases where the printing resolution of the ejection nozzles and the reading resolution of the scanner unit are close to each other (or a case where the reading resolution of the scanner unit is finer than the printing resolution of the ejection nozzles) or cases where the reading resolution of the scanner unit is coarser than the printing resolution of the ejection nozzles.
If the reading resolution of the scanner unit is coarser than the printing resolution of the ejection nozzles, then a plurality of lines are read in by one determination element of the scanner unit, and as a result of this, there is a possibility that the position of a nozzle suffering ejection failure cannot be identified. If the reading resolution of the scanner unit is not sufficiently large, then one pixel of the inspection sensor also include the next pixel of the image, and hence there is a possibility that a nozzle suffering ejection failure cannot be identified in the conveyance direction either.
In other words, if the length is insufficient in the paper conveyance direction, then this means that two patterns are read in by one determination element, and hence a nozzle suffering ejection failure cannot be identified.
Moreover, if there is change in the size of the ejected droplets or their direction of flight, in either the main scanning direction (the breadthways direction of the paper, X direction) or the sub-scanning direction (the conveyance direction, Y direction), or if the position of the ejection head is displaced with respect to the paper, the interval of the test pattern in the X direction which is set once, in other words, the number N in the 1-on N-off arrangement, becomes inappropriate, and there is a possibility that a plurality of ejected droplets can be included in one inspection pixel of the scanner unit. Even after the length of the test pattern in the Y direction has been set to a state whereby only the ejection from one nozzle is contained in one inspection pixel of the scanner unit, if deviation occurs in the droplet ejection size or direction or the position of the ejection head, then there is a possibility that the state changes to one where the droplet ejection from a plurality of nozzles is contained in one inspection pixel of the scanner unit and hence the nozzles cannot be identified.